Regulation
Since C3b is free and abundant in the plasma, it can bind to either a host cell or a pathogen surface. To prevent complement activation from proceeding on the host cell, there are several different kinds of regulatory proteins that disrupt the complement activation process:
- Complement Receptor 1 (CR1 or CD35) and DAF (decay accelerating factor also known as CD55) compete with Factor B in binding with C3b on the cell surface and can even remove Bb from an already formed C3bBb complex
- The formation of a C3 convertase can also be prevented when a plasma protease called Factor I cleaves C3b into its inactive form, iC3b. Factor I requires a C3b-binding protein cofactor such as complement factor H, CR1 and Membrane Cofactor of Proteolysis (MCP or CD46)
- Complement Factor H can inhibit the formation of the C3 convertase by competing with factor B for binding to C3b; accelerate the decay of the C3 convertase; and act as a cofactor for Factor I-mediated cleavage of C3b. Complement factor H preferentially binds to vertebrate cells (because of affinity for sialic acid residues), allowing preferential protection of host (as opposed to bacterial) cells from complement-mediated damage.
- CFHR5 (Complement Factor H-Related protein 5) is able to bind to act as a cofactor for Factor I, has decay accelerating activity and is able to bind preferentially to C3b at host surfaces.
Read more about this topic: Alternative Complement Pathway
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